Grilled Flank Steak with Sauteed Portabella and Cremini Mushrooms

Pat steak dry and cut crosswise at a 45° angle into 4 equal pieces. Season steaks with salt and pepper and transfer to a large resealable heavy-duty plastic bag. Chop shallots and in a bowl stir together with vinegar, sugar, and soy sauce. Pour marinade into bag. Marinate steaks, covered and chilled, turning bag over at least once, at least 2 hours and up to 2 days.

Prepare grill.

Remove steaks from bag and discard marinade. Grill steaks on a lightly oiled rack set 5 to 6 inches over glowing coals 7 to 9 minutes on each side for medium-rare. (Alternatively, grill steaks in 2 batches in a hot lightly oiled well-seasoned ridged grill pan over moderately high heat.) Transfer steaks as cooked to a cutting board and let stand 10 minutes.

Cut steak at a 45° angle across grain into thin slices. Serve steak with sautéed mushrooms.